Mild traumatic brain injury (TBI), or concussion, is epidemic in the United States affecting at least half a million adolescent athletes annually. There is now a greater awareness of the possibility of long term neurological consequences of repeated concussions suffered during a time when the brain is still developing, including the possibility of long term cognitive and other neurological deficits. Unfortunately, almost nothing is known about the specific mechanisms leading to brain injury after repeated mild TBI, and no specific therapy other than rest exists to reduce long-term cognitive and other sequelae. To begin to address these knowledge gaps, we developed a repetitive closed head injury (rCHI) model in adolescent mice that produces sub-concussive biomechanical forces, mild histopathology, and long-term deficits in learning and memory, brain connectivity, cerebrovascular reactivity, and respiratory and heart rate reactivity to carbon dioxide challenge. Our goals are to examine the relationship between the vulnerable period to repetitive injuries and development of neurological and physiological deficits in adolescent mice, use fMRI and tests of respiratory and heart rate reactivity to carbon dioxide challenge to predict the brain's vulnerable period to further injury resulting in permanent neurological deficits, and test the hypothesis that endothelial interleukin-1 signaling mechanisms in part mediate outcome in the rCHI model, with the following Specific Aims: Aim 1: Characterize the neurological deficits of rCHI using a focused battery of tests with known brain circuitry; define electrophysiological correlates of neurological dysfunction; and perform axon tracing and cFos immunohistochemistry to examine circuit integrity in injured male and female mice. Aim 2: Using fMRI/BOLD, characterize the effects of single and 3 hit daily (3HD) vs. 3 hit weekly (3HW) CHI on cerebrovascular reactivity (CVR) at acute and chronic time points after injury. Characterize respiratory and heart rate reactivity to inhaled CO2 and test the hypothesis that abnormal reactivity of cerebral blood flow and respiratory and heart rate are physiological biomarkers of closed head injury (CHI) that can be used to predict safe rest interval between repeated CHIs. Aim 3: Test the hypothesis that brain endothelial IL-1 signaling mediates postinjury cognitive deficits in adolescent mice using genetic and pharmacological tools. The proposed studies would lay the groundwork for future mechanistic/treatment studies of repetitive concussive TBI in adolescents.